Illicit drug use and abuse remains a societal burden affecting 9.2% of the population ages 12 and over and costs the nation hundreds of millions of dollars every year. It is well known that the nucleus accumbens (NAc) is a brain region important in reward and motivation and is altered by drugs of abuse. This structure is primarily composed of two types of medium spiny neurons (MSN) which differentially modulate behavior. However, despite the high prevalence and cost of drug abuse, mechanisms underlying synaptic and behavioral alterations within the NAc occurring following drug exposure remain poorly understood. Though recent research identifies the immune system as a regulator of neuronal physiology and behavior, examination into how the innate immune system affects drug-induced changes in NAc synaptic physiology has not been performed. Preliminary experiments suggest that toll-like receptor 4 (TLR4), a pattern recognition molecule of the innate immune system, is involved in drug-behavior and synaptic physiology. In the brain, TLR4 is primarily associated with microglia. The studies outlined in this proposal will test the hypothesis that TLR4 contributes to NAc-related behaviors and synaptic physiology. In Specific Aim 1, a battery of drug-reward and non-drug behavioral assays will be performed on TLR4-knockout (TLR4.KO), tamoxifen-inducible conditional microglial TLR4.KO (imTLR4.KO), and wild-type (WT) mice. The imTLR4.KO animals permit study of TLR4 without developmental confounds. These experiments assess the contribution of TLR4 on drug- and non-drug behavior. In Specific Aim 2, whole-cell electrophysiology will be performed on acute brain slices taken from WT, TLR4.KO, and imTLR4.KO mice expressing a fluorescent reporter protein on D1 dopamine receptor expressing MSNs. The reporter protein allows discrimination between the two types of MSNs. Using this technique, the contribution of TLR4 on NAc MSN basal synaptic properties and synaptic plasticity will be examined in a cell-type-specific manner. Recordings will be analyzed for differences in quantal release, AMPA receptor/NMDA receptor ratios, ionotropic glutamate receptor stoichiometry, and presynaptic release probability. Synaptic plasticity experiments will examine TLR4-dependent differences in NMDA receptor- dependent long-term depression. In Specific Aim 3, the whole-cell electrophysiology experiments described in Specific Aim 2 will be repeated in the same groups of animals abstained from chronic cocaine exposure. This will examine how TLR4 affects drug-induced changes in synaptic plasticity and basal properties. These experiments will advance our understanding of how the innate immune system and neuronal components interact to shape drug-induced behavior and synaptic physiology. Better knowledge of these mechanisms is a necessary step towards improved therapies to combat drug abuse.